Centrifugation is used to separate components in samples of biological materials according to the mass, density, and other sedimentation characteristics of the components, upon application of a gravitational field. Centrifuge tubes must withstand the applied gravitational forces and are typically cylindrical with a flat or slightly rounded bottom. The shape of the walls of the centrifuge tube imparts separation characteristics to the sample within the tube. Additional separation characteristics are provided by the temperature and chemical composition of the sample, the material of which the tube is constructed, and the relative masses and densities of the sample components.
In many clinical or research applications, a portion of the sample must be recovered for further analysis or purification. Where the portion or fraction to be recovered is at the top of the tube, isolation can be relatively simple, for example by aspiration or pipetting. However, where the fraction of interest is in a middle or lower fraction of the tube, recovery of that fraction may be complicated by possible re-contamination with other tube contents.
Attempts to overcome this problem include puncturing a side wall of the tube, for example, with a syringe, to remove the fraction of interest, as described in Sambrook et al, Molecular Cloning, 2nd Edition. This technique has inherent safety risks associated with the puncturing process and requires a readily puncturable tube. Additionally, this procedure is time consuming and also frequently leads to loss of sample. Other tubes have stoppers, plugs, or specialized accessories to disgorge the lowermost portion of the tube contents into another sample container, as described, for example in U.S. Pat. No. 4,861,477. However, where the sample fraction of interest is in the middle of the tube, recovery of the sample using these methods may be difficult and time consuming. Further, these techniques do not entirely obviate the problems of contamination of the sample with other biological fractions, or contamination of the work place with the blood sample itself.
Therefore, it would be desirable to have a centrifuge tube and apparatus which permits centrifugation of blood samples at speeds sufficient to separate cells according to their density and sedimentation characteristics, and which facilitates extraction of a blood sample with a minimum of cross contamination. Surprisingly, the present invention satisfies these and other related needs.